Various recombinant and nonrecombinant Alpha and Gamma interferons have been tested in Phase I trials in cancer patients in order to study the toxicity, antitumor effects, immunomodulatory effects and pharmacokinetics of these preparations. The initial Phase I trials employed highly purified recombinant leukocyte A interferon and human Namalva cell lymphoblastoid interferon and have been previously reported. We have recently completed 3 phase I studies with recombinant and nonrecombinant Gamma interferons. Toxicity for each of these preparations was similar to Alpha interferons with a flu-like syndrome as well as minor hematologic toxicity, primarily decreased leukocytes. Dose-dependent serum levels were measured using both a biologic assay and enzyme-linked immunosorbent assay for the recombinant Gamma interferon. No antitumor responses were seen.